It has been known in the prior art to use protective devices for the protection of electrical instruments such as rechargeable battery units, electric motors, etc., wherein such devices maintain a "self hold" or "contacts open" states by the use of a positive temperature coefficient (PTC) element in the device for providing heat generation even when the contacts are open and the current through them is shut off.
Typically, this type of protection device is constructed by having two terminals with a snap acting bimetal member with a movable contact fixed to one of the terminals. The other terminal has a fixed contact attached to it which is positioned such that the movable contact on the bimetal member generally is in contact with the fixed contact. Additionally, positioned between the two terminals a PTC element is held in place by an electrically conductive spring member. Upon an overcurrent and overheating occurrence, the bimetal is heated and snaps to a position in which the movable and fixed contacts are no longer in contact with each other and thus the voltage between the terminals is impressed completely across the PTC element. The PTC element then becomes heated to a fixed temperature which holds the contact in the "open state". In such a device, an electroconductive spring plate has been required for electrically holding the PTC element between the terminals. This structure is more expensive to produce and increases the overall thickness of the device.